Over Sea Under Stone seems to revolve heavily around the legends of King Arthur
and his knights, other themes present include the Holy Grail and Merlin. The way Susan Cooper presents these legends isn’t entirely unique but blends well with the main theme of the story which causes the reader to easily accept these ideas, concepts and legends. I don’t particularly enjoy how Susan Cooper uses the Holy Grail to become a concept of good and evil because ultimately good and evil did exist long before Jesus was born and the Holy Grail was drunk from.

I see what you mean, we never did find out what Mr Hastings would do with it if he got it.